High pressure reactors, such as tubular reactors and autoclaves, are used for the polymerization of ethylene at high pressure, for example, pressures of over 1000 bar, and up to 3000 bar or higher. In such processes, fresh ethylene from an ethylene supply is compressed to reactor pressure, typically by a combination of a primary compressor which compresses the ethylene to an intermediate pressure, say, around 300 bar, and a secondary compressor which compresses the ethylene from that intermediate pressure up to the final reactor pressure. The ethylene is then combined, in the reactor, with initiators and any comonomers and is polymerised to give a mixture comprising principally polymer and unreacted monomer. That mixture leaves the reactor through a valve, generally referred to as a high pressure let down valve, and then enters a separation system in which unreacted monomer is separated from the polymer and recycled back to the suction of the secondary compressor where it is combined with the fresh ethylene from the primary compressor.
Various forms of separation system are known. One such known separation system includes two separation vessels arranged in series. The first separation vessel, sometimes referred to as the high pressure vessel, has an inlet for the product mixture coming from the high pressure let down valve, an outlet for the separated unreacted monomer gas (referred to as “off gas”) and an outlet in the bottom of the vessel for the polymer. That polymer, which still contains, say, 30 to 40 wt % of ethylene, passes from the outlet of the first vessel through a conduit into the second vessel, often referred to as the low pressure vessel, where almost all of the remaining ethylene separates off and is removed through an upper outlet as off gas leaving the molten polymer to flow through an outlet in the bottom of the vessel. Typically, the high pressure vessel will operate at a pressure such that the off gas can be returned, via a recycle system, to the suction of the secondary compressor. The low pressure separator operates at a much lower pressure, and the off gas from the low pressure separator must be compressed in a further compressor (known as a “purge compressor”) before being sent to the secondary compressor.
The molten polymer leaving the separation system is generally extruded and cooled to give a solid product, typically in pellet form, which is sent for storage or to other product handling facilities. The polymer product typically retains a small amount of residual ethylene, which slowly leaks out of the pellets. The product storage facilities must therefore be well ventilated to prevent a build up of ethylene. However, the venting of organic compounds such as ethylene to the atmosphere is now recognised as being undesirable, and is subject to regulatory control in many parts of the world.
The polymerization of ethylene is an exothermic process which generates heat. Usually, some of that heat energy is recovered from the recycled ethylene and used to generate steam. In spite of that saving, the electrical energy required for the compression of the ethylene to reactor pressure represents a significant proportion of the cost of the final product and therefore it is desirable for both economic and environmental reasons to maximise the amount of polymer produced per unit energy consumed by the compressors.
High pressure polymerization reactors generally have lifetimes of 20 or 30 years, or even longer. For many years now, great efforts have been made to increase the productivity of existing facilities by “debottlenecking”. However, there remains a need to increase the capacity of existing high pressure polymerization plants.
U.S. Pat. No. 4,342,853 describes an intermediate pressure separator. However this document does not teach that the off gas from at least one separation vessel downstream of the first separation vessel is returned to a location downstream of the high pressure let down valve and upstream of the first separation vessel.
U.S. Pat. No. 4,082,910 uses the residence time in the separators to control product properties; however there is no teaching to use an intermediate pressure separator.
U.S. Pat. No. 4,215,207 describes the use of a jet pump to pull in gas from the recycle outlet into the outlet stream of the reactor without use of a separator operating at a pressure intermediate a higher and lower pressure separator.
It would be desirable to have apparatus and processes which at least partially met one or more of the above-mentioned needs.